Sealed honeycomb structure and method of producing the same

ABSTRACT

There is provided a plugged honeycomb structure  1  including: partition walls  2  disposed so as to form a plurality of cells  3  extending from one end face  42  to the other end face  44  in the axial direction, and plugged portions  4  disposed so as to plug the cells  3  at one of the end faces, and a production method thereof. In this honeycomb structure  1 , the plugged portions  4  and partition walls surrounding the plugged portions are unitarily formed. The production method includes a forming step, a plugging step for filling a plugging material, and a firing step. The plugging material contains solid particles capable of unitarily joining with at least one kind of solid particles contained in a forming raw material in a firing step. A ratio of a dimensional change (%) upon forming the partition walls out of the partition wall-forming material to a dimensional change (%) upon forming the plugged portions out of the plugging material is controlled to be within the range of 0.7% to 1.3 in the firing step. There is provided a plugged honeycomb structure having further improved strength, adhesion, and thermal shock resistance between partition walls and plugged portions and a production method thereof.

TECHNICAL FIELD

The present invention relates to a sealed or plugged honeycomb structureusable for an exhaust gas purification filter, a water treatment filter,and a separation membrane filter and a production method thereof. Inparticular, the present invention relates to a plugged honeycombstructure having high strength, adhesion, and thermal shock resistancebetween partition walls and plugged portions.

BACKGROUND ART

When a honeycomb structure is used as an exhaust gas purificationfilter, as shown in FIGS. 9( a) to 9(c), the honeycomb structure isgenerally used in the form of a plugged honeycomb structure 1 providedwith porous partition walls 2 disposed in such a manner that a pluralityof cells 3 are extending from one end face 42 to the other end face 44in an axial direction and plugged portions 4 disposed in such a mannerthat each of the cells 3 is plugged at one of the two end faces. Byemploying such a form, fluid to be treated flowing into cells from oneend face 42 passes through the porous partition walls 2 and isdischarged from the other end face 44 via other cells 3. At this time,the partition walls 2 function as a filter to trap particulate matter orthe like.

Such a plugged honeycomb structure can generally be produced byproducing a honeycomb structure with no plugged portion and subsequentlyforming plugged portions in predetermined cells. However, since variouskinds of stress is prone to be applied on, for example, an interfaceportion 5 between the plugged portions 4 and the partition walls 2 shownin FIG. 10, a crack is prone to be caused in this portion. In addition,particulate matter such as soot in exhaust gas sometimes leaks out froma gap between a partition wall and a plugging portion during use of aplugged honeycomb structure as a diesel particulate filter (DPF), andwhen a DPF is washed by water with high pressure or compressed air inorder to discharge outside the DPF particulate matter such as an ashcomponent or a soot component accumulated inside the DPF after the DPFis used for an exhaust gas treatment, a plugged portion is sometimesdetached from partition walls to move, and when a plugged portion movestoo much, it is sometimes detached from the DPF.

For such problems, there is disclosed a honeycomb filter having adifference in thermal expansion coefficient between a honeycombstructure and plugged portions within a predetermined range, comprising:a mechanism of bonding with a plugging material filling up open porespresent in partition walls of a honeycomb structure dried and firedafter being formed, and a mechanism of engaging with through holes, andbeing characterized by substantially not interposing a reaction phase ina contact portion of the ceramic honeycomb structure and the pluggedportion (see, for example, JP-A-57-42316).

As a plugging method in which firing is performed after plugging ahoneycomb formed body before firing, there has been proposed a pluggingmethod using ceramic particles and an auxiliary for fluidizing theceramic particles as a plugging material and being characterized in thatthe auxiliary has the property of not dissolving again the bindercontained in the honeycomb formed body (see JP-A-2002-173381). Thismethod aims to suppress deformation or dissolution of a honeycomb formedbody upon immersing the honeycomb formed body in a plugging material.

DISCLOSURE OF THE INVENTION

The present invention is characterized by providing a plugged honeycombstructure having further improved strength, adhesion, and thermal shockresistance between partition walls and plugged portions and a productionmethod thereof.

In the present invention, the aforementioned conventional techniqueswere investigated in detail, and, as a result, the following knowledgewas obtained. That is, in the aforementioned techniques, because of lowaffinity between the plugged portions and partition walls surroundingthe plugged portions, an interface was present in an interface portion 5between the plugged portions 4 and the partition walls 2 surrounding theplugged portions 4 and had week intrinsic bonding force. Therefore, inorder to maintain the plugged portions in cells, it was necessary for avolume of the plugged portions to expand to some extent, therebypress-widening the peripheral partition walls to utilize reactive forcethereof. Therefore, a residual stress is generated in the peripheralpartition wall as a side effect to lower the strength against a thermalstress.

Therefore, the present inventors found out that by improving affinitybetween plugged portions and partition walls surrounding the pluggedportions, and further unitarily joining plugged portions and partitionwalls surrounding the plugged portions, intrinsic bonding force can beimparted to the portion between plugged portions and partition wallssurrounding the plugged portions; and strength, adhesion, and thermalshock resistance sufficient for maintaining the structure even undersevere conditions can be imparted to the structure. They also found outthat plugged portions and partition walls surrounding the pluggedportions can be unitarily joined by a specific method.

That is, according to the present invention, there is provided a pluggedhoneycomb structure comprising:

partition walls disposed so as to form a plurality of cells extendingfrom one end face to the other end face in an axial direction, and

plugged portions disposed so as to plug the cells at one of the endfaces,

wherein the plugged portions and said partition walls surrounding saidplugged portions are unitarily formed.

In the present invention it is preferable that a length of the pluggedportions in a longitudinal direction of the cells is ten times as longas a cell pitch or less and that the length of the plugged portions inthe longitudinal direction of the cells is two times as long as athickness of the partition walls or more.

According to the present invention, there is further provided a methodfor producing a plugged honeycomb structure, comprising:

a forming step of forming a forming raw material containing a solidparticle (A) into a honeycomb formed body provided with a partitionwall-formed body disposed so as to form a plurality of cells extendingfrom one end face to the other end face in an axial direction,

a plugging step of filling a plugging material containing a solidparticle (B) into end portions of cells of the honeycomb formed body toform a plugged honeycomb formed body, and

a firing step of firing the plugged honeycomb formed body to form aplugged honeycomb structure provided with partition walls formed out ofthe partition wall-formed body and plugged portions formed out of theplugging material;

wherein the solid particle (A) and the solid particle (B) can unitarilybe formed mutually in the firing step, and

a difference between rate of dimensional change (%) upon forming theplugged portions out of the plugging material and rate of dimensionalchange (%) upon forming the partition walls out of the partitionwall-forming material is controlled to be 7% or less within thetemperature range of the firing in the firing step.

In the present invention, the method preferably further comprises acooling step of cooling the plugged honeycomb structure, and a ratio ofthermal expansion coefficient of the plugged portions to thermalexpansion coefficient of the partition walls is made to be preferably0.3 to 3.0%, further preferably 0.5 to 2.0, particularly preferably 0.7to 1.4 in the temperature range of the cooling in the cooling step. Inaddition, it is preferable that the plugging step includes a step offilling the plugging material from one end face of the honeycombstructure with pressurizing inside the cells from the other end face andthat the forming raw material contains a binder, and the pluggingmaterial contains a binder compatible with the binder contained in theforming raw material. It is also preferable that the forming rawmaterial contains a binder, and after the forming step, an end face ofthe honeycomb formed body is heated at 200° C. or more to remove atleast a part of the binder, followed by conducting the plugging step atthe end face. It is also preferable that the forming raw material is aslurry containing a dispersion medium (C), and the plugging material isa slurry containing a dispersion medium (D) compatible with thedispersion medium (C) and further preferable that the honeycomb formedbody is dried, subsequently, the partition wall-formed body isimpregnated with the dispersion medium (C) in the vicinity of an endface, followed by conducting the plugging step at the end face.Alternatively, it is also preferable that the dispersion medium (C) andthe dispersion medium (D) are hydrophobic, and the plugging step isconducted under the condition that temperature of the partitionwall-formed body is higher than the normal temperature in the vicinityof an end portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1( a) is a schematic perspective view showing an embodiment of aplugged honeycomb structure of the present invention, FIG. 1( b) is aschematic partially enlarged plan view showing b portion of FIG. 1( a),and FIG. 1( c) is a schematic parallel sectional view of FIG. 1( a).

FIG. 2 is a schematic partially enlarged parallel sectional view showinganother embodiment of a plugged honeycomb structure of the presentinvention.

FIG. 3 is a diagram schematically showing a relation between temperatureand change in dimensions in a production method of a plugged honeycombstructure of the present invention.

FIG. 4 is a diagram schematically showing a relation between temperatureand change in dimensions in a present production method of a pluggedhoneycomb structure.

FIG. 5 is a schematic parallel sectional view for explaining a pluggingstep in a production method of the present invention.

FIG. 6 is a schematic parallel sectional view for explaining apreferable plugging step in a production method of the presentinvention.

FIG. 7 is an enlarged parallel sectional photograph of a pluggedhoneycomb structure produced by a production method of the presentinvention.

FIG. 8 is an enlarged parallel sectional photograph of a pluggedhoneycomb structure produced by a conventional production method.

FIG. 9( a) is a schematic perspective view showing a conventionalplugged honeycomb structure, FIG. 9( b) is a schematic partiallyenlarged plan view showing b portion of FIG. 9( a), and FIG. 9( c) is aschematic parallel sectional view of FIG. 9( a).

FIG. 10 is a schematic partially enlarged sectional view showing aconventional plugged honeycomb structure.

BEST MODE FOR CARRYING OUT THE INVENTION

A plugged honeycomb structure and a production method thereof of thepresent invention will hereinbelow be described in detail on the basisof specific embodiments. However, the present invention is by no meanslimited to the following embodiments. Incidentally, in the followingdescription, a section orthogonal with an axial direction (e.g., axialdirection shown in FIG. 1) is referred to as an orthogonal section, anda section parallel with an axial direction is referred to as a parallelsection.

As shown in FIGS. 1( a) to 1(c), a plugged honeycomb structure 1 of thepresent invention is provided with partition walls 2 disposed so as toform a plurality of cells 3 extending from one end face 42 to the otherend face 44 in the axial direction and plugged portions 4 disposed so asto plug the cells 3 at one of the end faces.

An important characteristic of a plugged honeycomb structure of thepresent invention is that the plugged portions 4 and the partition wall2 surrounding the plugged portions are unitarily formed as shown in FIG.2. By the unitary formation, even in the case that an excessive force isapplied from a longitudinal direction (axial direction) of cells to theplugged portions, a possibility that the plugged portions come off inthe direction can be reduced. In addition, since the plugged portionscan be held in the cells with sufficient strength even without usingvolume expansion of the plugged portions, a residual stress is not proneto be generated on partition walls surrounding the plugged portions,thereby improving thermal shock resistance. Therefore, even in the casethat the honeycomb structure is put under severe temperature conditions,a crack is not prone to be caused in the interface portion 5 andpartition walls 2 surrounding the interface portion. In addition, apossibility that an untreated fluid leaks out from an interface portion5 between a plugged portion 4 and a partition wall 2 surrounding theplugged portion is markedly reduced.

In the present invention, “plugged portions and partition wallssurrounding the plugged portions are unitarily formed” means there is nostructural interface between the plugged portion 4 and the partitionwall 2 surrounding the plugged portion. It means, for example, in thecase that a parallel section in a portion of the partition wall 2surrounding the plugged portion 4 is observed, no interface line isobserved and one phase is present in such a manner that it extends overboth the plugged portion 4 and the partition wall 2 surrounding theplugged portion as shown in FIG. 2. For example, since a reaction phaseis not interposed substantially at the interface in a relation of aplugged portion and a partition wall surrounding the plugged portiondisclosed in JP-A-57-42316, a structural interface is observed betweenthe plugged portion and the partition wall surrounding the pluggedportion even with the plugged portion entering an open pore of thepartition wall. In contrast, since plugged portions and partition wallssurrounding the plugged portions are unitarily formed by, for example,melting by firing in a plugged honeycomb structure of the presentinvention, such a structural interface is not observed.

In the present invention, there is no particular limitation to a lengthy of the plugged portions 4 in a longitudinal direction (axialdirection) of the cells 3 shown in FIG. 2. However, since a pluggedhoneycomb structure of the present invention has high bonding forcebetween the plugged portions and the partition walls surrounding theplugged portion, it is not required to make the length y of the pluggedportions long. In addition, as the length y of the plugged portionsbecome larger, a thermal stress concentrates more easily to lowerthermal shock resistance when the plugged honeycomb structure has hightemperature. Particularly, when bonding force between plugged portionsand partition walls surrounding the plugged portions like a pluggedhoneycomb structure of the present invention, rigidity is high in theplugging portions of the plugged honeycomb structure, and when thelength y of the plugged portion is made large in such cases, thermalshock resistance is prone to be lowered. Therefore, in the presentinvention, the length y of the plugged portion is preferably 10 times orless, more preferably 5 times or less, of a cell pitch. Here, a cellpitch means a length shown by Z in FIG. 2 and a length of one cell as arepeated unit. In addition, in the case that a plugged honeycombstructure of the present invention is used as a filter, it is notpreferable that the structure has too large length y of a pluggedportion because a partition wall has a small filtration area. From thisviewpoint, the length y of a plugged portion is preferably 10% or less,more preferably 5% or less, of a length x in a longitudinal direction ofa cell shown in FIG. 1( a). The technique of unitarily forming thepartition walls and the plugged portions of the present invention makesit possible to shorten the length of the plugged portions and to improvethermal shock resistance, thereby realizing reduction in pressure loss.

When the length y of a plugged portion is too small, even a pluggedhoneycomb structure of the present invention sometimes has insufficientstrength against pressure from a longitudinal direction of cells.Therefore, the length y of a plugged portion is preferably 2 times ormore, more preferably 4 times or more, the thickness of partition wallsW. However, in the case that the pressure from a longitudinal directionis small enough to be ignored, the above lower limit of a length of aplugged portion does not apply to this case. For example, as an extremecase, it is possible to make the length of a plugged portionsubstantially zero by attaching a thin sheet formed by a pluggingmaterial to an inlet of cells in an end face of a honeycomb structureand unitarily forming the plugged portion with the partition walls inthe present invention.

In a plugged honeycomb structure of the present invention, there is noparticular limitation to a shape or a material as long as it haspartition walls 2 disposed so as to form a plurality of cells 3extending from one end face 42 to the other end face 44 and pluggedportions 4 disposed so as to plug the cells 3 at one of the end faces,for example, as shown in FIGS. 1( a) to 1(c). A shape of an orthogonalsection of a plugged honeycomb structure can suitably be selectedaccording to use or a place where the structure is placed and is, forexample, a circle, an ellipse, a racetrack shape, or a quadrangle. Ashape of an orthogonal section of a cell can be, for example, a polygonsuch as a triangle, a quadrangle, and a hexagon, a circle, or asubstantial circle such as an ellipse. A cell density can be made to be,for example, 6 to 2000 cell/inch² (0.9 to 311 cell/cm²), preferablyabout 50 to 1000 cell/inch² (7.8 to 155 cell/cm²). In addition, as shownin FIGS. 1( a) to 1(c), it is preferable that adjacent cells haveplugged portions 4 on mutually opposite end faces, and the pluggedportions are disposed so that each of the end faces 42, 44 forms acheckerwise pattern. In addition, though there is no limitation to amaterial for plugged portions and partition walls, ceramic or metal ispreferable from the view point of thermal resistance, and particularly,ceramic is preferable. Further, the plugged portions and partition wallssurrounding the plugged portions preferably contain the same metal orceramic in order that the plugged portions and partition wallssurrounding the plugged portions may effectively be unitarily formed. Inaddition, a plugged honeycomb structure of the present invention ispreferably provided with an outer peripheral wall 7 surrounding an outerperiphery of the partition walls. When a plugged honeycomb structure ofthe present invention is used as a catalyst support or a filter, thepartition walls and the outer peripheral wall are preferably porous.

Next, a production method for suitably producing a plugged honeycombstructure of the present invention will be described. A productionmethod of the present invention includes a forming step for forming aforming raw material containing a solid particle (A) into a honeycombformed body, a plugging step of plugging cells with a plugging materialcontaining a solid particle (B), and a firing step of firing the pluggedformed body to obtain a plugged honeycomb structure.

The important characteristics of the present invention are that thesolid particle (A) contained in the forming raw material and the solidparticle (B) contained in the plugging material are solid particlescapable of being unitarily joined mutually in the firing step and that adifference between rate of dimensional change (%) upon forming theplugged portions out of the plugging material and rate of dimensionalchange (%) upon forming the partition walls out of the partitionwall-forming material is controlled to be 7% or less, preferably 5% orless, more preferably 3% or less, within the temperature range of thefiring in the firing step. Here, a difference between rates ofdimensional change (%) means a value obtained by subtracting the smallerrate of dimensional change (%) from the larger rate of dimensionalchange (%) between the two rates of dimensional change (%). For example,when a rate of dimensional change (%) is 4 (%), and the other rate ofdimensional change (%) is 10 (%), the difference in rate of dimensionalchange (%) is 6 (%).

By the production method, the plugged portions and the partition wallssurrounding the plugged portions can unitarily be formed. For example,at least a part of a solid particle (B), for example, a ceramic particlecontained in a plugging material for forming plugged portions and atleast a part of a solid particle (A), for example, a ceramic particlecontained in a forming raw material for forming partition wallssurrounding the plugged portions are melted in the firing step to beunitarily formed, thereby the plugged portions and the partition wallssurrounding the plugged portions can unitarily be formed.

However, the plugged portions and the partition walls surrounding theplugged portions cannot unitarily be formed only by unitarily joiningthe solid particle (A) contained in the forming raw material and thesolid particle (B) contained in the plugging material. That is, by thedifference between a dimensional change caused when the pluggingmaterial is changed into plugged portion by firing and a dimensionalchange caused when the forming raw material is changed into thepartition walls by firing, stress is applied to the interface portionbetween the range of the plugged portions and the range of the partitionwalls during firing, and thereby the unitary formation is hindered.Therefore, by controlling the absolute value of a difference betweenrate of dimensional change (%) upon forming the plugged portions out ofthe plugging material and rate of dimensional change (%) upon formingthe partition walls out of the partition wall-forming material to be 7%or less, preferably 5% or less, more preferably 3% or less, within thetemperature range of the firing in the firing step, at least a part ofthe solid particles (A) and (B) can be unitarily joined, and the pluggedportions and the partition walls surrounding the plugged portions can beunitarily formed.

FIGS. 3 and 4 are diagrams showing examples of relations between rate ofdimensional change (%) and temperature of partition walls and pluggedportions in a conventional production method and a production method ofthe present invention, respectively. By controlling a difference in rateof dimensional change shown in these diagrams to be within the aboverange, the plugged portion and the partition walls surrounding theplugged portions can be unitarily formed. Though the difference in rateof dimensional change is ideally zero, it is not necessarily zero asshown in FIG. 4. When it is not zero, a residual stress is generated.The influence of the residual stress to strength, thermal shockresistance, and the like of a plugged honeycomb structure variesdepending on a contact area of the plugged portions to the partitionwalls, the size of the plugged honeycomb structure itself, and the like.The contact area varies depending on the size of an orthogonal sectionof the cells and a length y of the plugged portions. Therefore, it ispreferable to rationalize the difference in rate of dimensional changebetween two portions to be within the above range in consideration of alevel of stress acting on the plugged portions under conditions ofpractical use.

In order to control the difference in rate of dimensional change to bewithin the above range, it is preferable, for example, to make aparticle size distribution of solid particles contained in the formingraw material close to that of solid particles contained in the pluggingmaterial, further to make it almost the same, or to adjust atemperature-rising speed. Incidentally, the dimensional change heremeans any of the dimensional changes caused by contraction or expansion.However, in firing solid particles such as ceramic particles to form afired body, most of the changes are caused by contraction. For example,in the case of producing a cordierite-based plugged honeycomb structure,contraction increases at 1000° C. or more where sintering of ceramicparticles starts as shown in FIGS. 3 and 4. Therefore, it is importantto control a difference in contraction percentage at 1000° C. more to bewithin the above range. Incidentally, the period for controlling theabsolute value of a difference between rate of dimensional change (%)upon forming the plugged portions out of the plugging material and rateof dimensional change (%) upon forming the partition walls out of thepartition wall-forming material to be 7 or less, preferably 5 or less,more preferably 3 or less, within the temperature range of the firing isthe period from the start of firing to the completion of maintaining thehighest temperature.

In the present invention, it is preferable that the method furtherincludes a cooling step of cooling the plugged honeycomb structure afterthe firing step, and that a ratio of thermal expansion coefficient ofthe plugged portions to thermal expansion coefficient of the partitionwalls is controlled to be 0.3 to 3.0, preferably 0.5 to 2.0, morepreferably 0.7 to 1.4 in the temperature range of the cooling in thecooling step. In the cooling step, high temperature of a pluggedhoneycomb structure in the firing step is returned to the normaltemperature. At this time, the plugged honeycomb structure generallycontracts. Accordingly, a too large difference in dimensional changebetween the plugged portion and the partition walls surrounding theplugged portions causes separation of unitarily joined solid particlesand hinders unitary formation of the plugged portions and the partitionwalls surrounding the plugged portions. Therefore, by controllingthermal expansion coefficients of both the plugged portions and thepartition walls to be within the above range, a plugged honeycombstructure having unitarily formed plugged portions and partition wallssurrounding the plugged portions can suitably be obtained. For example,since the highest temperature is 1400° C. or more when a cordieriteplugged honeycomb structure is produced, thermal expansion coefficientin the range from 1400 to 800° C. is more important. Also because of thelarge rate of change in thermal expansion coefficient, the ratio ofthermal expansion coefficient of partition walls to the thermalexpansion coefficient of plugged portions is preferably within the rangeof 0.7 to 1.4. For a thermal expansion coefficient of plugged portions,there can be used a value obtained by forming a plate having dimensionsof about 6 cm×6 cm (sides)×6 mm (thickness) with a slurried pluggingmaterial, drying and firing the plate, cut out a square prism-shapedsample having dimensions of about 3 mm×3 mm (sides)×50 mm (length) fromthe plate, and measuring a thermal expansion curve (a rate ofdimensional change (ΔL/L) per unit temperature change (° C.) under afixed pressure) within the temperature range from 40° C. to 1400° C.with a thermal expansion measuring apparatus. For a thermal expansioncoefficient of partition walls of a honeycomb structure, there can beused a value obtained in the manner similar to the case of the pluggingmaterial by cut out a square prism-shaped sample having dimensions ofabout 3 mm×3 mm (in a direction toward inside the partition walls)×50 mmin a direction perpendicular to the cell passages.

Next, each step is described. The forming step can be conducted asfollows: There is employed as a solid particle (A) a powder of amaterial selected from various ceramics such as cordierite, mullite,alumina, spinel, zirconia, silicon carbide, silicon carbide-cordieritebased composite material, silicon nitride, lithium aluminum silicate,and aluminum titanate, and metals such as Fe—Cr—Al based material, andcombinations thereof. To the solid particle (A) is preferably added abinder such as methyl cellulose or hydroxypropoxylmethyl cellulose, thenit is preferably to further add a dispersion medium (C), for example,water to prepare a forming raw material. This is made to be clay havingplasticity, and then, the clay is extruded to give a honeycomb shape,and thereby a honeycomb formed body having partition walls to form aplurality of cells extending from one end face to the other end face inan axial direction can be formed. Incidentally, to the forming rawmaterial may be added a desired additive as necessary. The additive maybe a binder, a dispersant for accelerating dispersion to a dispersionmedium, a pore former for forming pores, or the like. Examples of thebinder include hydroxypropylmethyl cellulose, methyl cellulose,hydroxyethyl cellulose, carboxylmethyl cellulose, poly(vinyl alcohol),poly(ethylene terephthalate), polyethylene, and glycerol. Examples ofthe dispersant include ethylene glycol, dextrin, fatty acid soap, andpolyalcohol. Examples of the pore former include graphite, flour,starch, phenol resin, and poly(ethylene terephthalate). These additivesmay be used alone or in combination according to the purpose. Inaddition, the forming raw material may include another solid particlebesides the solid particle (A). The solid particle is preferablyselected from the above examples of the solid particle (A).

In the plugging step, for example, a binder, a dispersion medium (D) orthe like is preferably added to a firing material containing a solidparticle (B) capable of unitarily joined with the solid particle (A)contained in the aforementioned forming raw material to prepare slurry,which is then put in a container 16 as shown in FIG. 5. Then, ahoneycomb formed body 10 whose cells except for the cells to be pluggedby the plugging material were masked with a masking member 12 at an endface 42 is immersed in the plugging material 6 to fill the pluggingmaterial in the end portions of the predetermined cells. The solidparticle (B) is preferably selected from the examples of the solidparticle (A) and more preferably the same kinds as the solid particle(A). The plugging material may contain another solid particle besidesthe solid particle (B). Another solid particle is preferably selectedfrom the above examples of the solid particle (A).

At this time, it is preferable to apply pressure inside cells from theside of the end face 44 which is not immersed in the plugging material.Applying pressure into inside cells makes permeation of the pluggingmaterial 6 easier as shown in FIG. 6, and thereby, unitary joining ofthe plugged portions and the partition walls surrounding the pluggedportions in the following firing step is made easier. The pressureapplied is preferably 0.1 to 10 kg/cm²G.

In addition, the forming raw material in the forming step preferablycontains a binder, and the plugging material preferably contains abinder capable of compatible with the binder contained in the formingraw material and more preferably contains the same binder. When theplugging material and the forming raw material contain such a binder,the plugging material can easily be permeate in partition wall-formedbody surrounding the plugged portions, and thereby unitary joining ofthe plugged portions and the partition walls surrounding the pluggedportions in the following firing step is made easier. A binder containedin the plugging material and the forming raw material may be selectedfrom the aforementioned examples of a binder.

It is generally preferable to remove the dispersion medium to someextent by drying a honeycomb formed body after the forming step andbefore the plugging step from the viewpoint of inhibiting deformation.In addition, when the forming raw material contains a binder, it ispreferable that an end face of a honeycomb formed body is heated at 200°C. or more, preferably 300° C. or more after the forming step to removeat least a part of the binder present in the vicinity of the end face,followed by the plugging step at the end face. By removing the binder inthe partition wall-formed body which will be in contact with theplugging material, a void is formed in the partition wall-formed body,and the plugging material can easily be permeated in the partitionwall-formed body in the periphery thereof. This makes unitary forming ofthe plugged portions and the partition walls surrounding the pluggedportions easier in the following firing step. In this case, it isnecessary to heat at least an end face, and the whole honeycomb formedbody may be heated.

In the case that the forming raw material is slurry containing adispersion medium (C), the plugging material is slurry containing adispersion medium (D) compatible with the dispersion medium (C). Alsobecause of this, the plugging material can easily be permeate in thepartition wall-formed body in the periphery thereof, and thereby makingunitary forming of the plugged portions and the partition wallssurrounding the plugged portions easier in the following firing step.For example, it is preferable that the dispersion medium (C) and thedispersion medium (D) are water or hydrophilic mediums or that thedispersion medium (C) and the dispersion medium (D) are hydrophobicmediums such as oil and wax. It is further preferable that thedispersion medium (C) and the dispersion medium (D) are the same medium.

In addition, it is preferable to dry a honeycomb formed body before theplugging step. For example, in the case that the dispersion medium (C)is water, almost all the dispersion medium (C) is removed by drying insome cases. In such a case, the dispersion medium (C) is made to becontained again in the partition wall-formed body in the vicinity of theend face, that is, in the portion, in contact with the plugging materialupon filling the plugging material, of the partition wall-formed body bymeans of spraying or the like, and the plugging step is conducted usinga plugging material containing a dispersion medium (D) compatible withthe dispersion medium (C) or a dispersion medium (D) which is the sameas the dispersion medium (C). The presence of the mutually compatibledispersion mediums in the plugging material and the partitionwall-formed body in the periphery thereof to some extent makes easierthe permeation and mixing of components contained in the pluggingmaterial and the partition wall-formed body, and this makes unitaryforming of the plugged portions and the partition walls surrounding theplugged portions easier in the following firing step.

In the case that the dispersion medium (C) and the dispersion medium (D)are hydrophobic, particularly, oil, wax, or the like, the plugging stepis preferably conducted by heating the partition wall-formed body in thevicinity of the end face, that is, the portion, in contact with theplugging material upon filling the plugging material, of the partitionwall-formed body to a predetermined temperature or higher. This makeseasier the permeation and mixing of components contained in the pluggingmaterial and the partition wall-formed body, and this makes unitaryforming of the plugged portions and the partition walls surrounding theplugged portions easier in the following firing step. Preferable heatingtemperature here is 100 to 300° C., and the plugging step is preferablyconducted within this temperature range.

The firing temperature can be conducted by raising temperature of theplugged honeycomb formed body formed in the above step in apredetermined firing atmosphere up to the temperature where the solidparticles used are sintered or reacted and maintaining the temperaturefor a predetermined time. Firing temperature and atmosphere may suitablybe changed depending on solid particles used, and those skilled in theart can select the firing temperature and atmosphere optimum for thesolid particles used. For example, in the case of using acordierite-forming raw material, firing can be conducted at the highesttemperature of 1400 to 1450° C. in the ambient atmosphere afterdegreasing in the ambient atmosphere; and in the case of using a siliconcarbide powder and a metallic silicon powder as a raw material, firingcan be conducted at the temperature of about 1550° C. in an Aratmosphere after degreasing in a N₂ atmosphere. For the firing, a singlefurnace or a continuous furnace such as a tunnel furnace is generallyused, and degreasing and firing can be conducted simultaneously here.Though it is considered that a temperature rising speed and a coolingspeed are not necessarily essential problems in the present invention,it is necessary to optimize a temperature rising speed and a coolingspeed to uniformalize a temperature distribution inside a productdepending the size of a product to be fired to realize uniform firingcontraction and cooling contraction, and a temperature rising speed anda cooling speed are very important factors in production.

The present invention is hereinbelow described with referring toExamples more specifically. However, the present invention is by nomeans limited to these Examples.

EXAMPLE AND COMPARATIVE EXAMPLE

First, to silica, kaolin, talc, and alumina as a cordierite-forming rawmaterial is added a foaming resin as a pore former. Further, a binder, adispersant, and water are added to the mixture, and the mixture iskneaded to obtain clay. Any pore former may be used as long as itscatters and disappears in the firing step. As a pore former, aninorganic substance such as carbon based substance, a high-molecularweight compound such as plastic material, or an organic substance suchas starch may be used alone or in combination.

It is preferable to use, as the main raw material for a forming rawmaterial for forming a honeycomb formed body, a composition containing,as a cordierite ceramic raw material excellent in thermal resistance andlow thermal expansibility, 0 to 20% by mass of kaolin having an averageparticle diameter of 5 to 10 μm, 37 to 40% by mass of talc having anaverage particle diameter of 15 to 30 μm, 15 to 45% by mass of aluminumhydroxide having an average particle diameter of 1 to 10 μm, 0 to 15% bymass of aluminum oxide having average particle diameter of 4 to 8 μm and10 to 20% by mass of fused silica or quartz having a particle diameterof 3 to 100 μm.

Next, using the kneaded clay-formed raw material, a honeycomb formedbody is formed by extrusion forming and dried. To 100 parts by mass of araw material powder containing the aforementioned main raw material andadditives added as necessary, about 10 to 40 parts by mass of water isadded, and the mixture is kneaded to obtain a mixture having plasticity.The mixture having plasticity is kneaded with a vacuum kneader to give araw material for forming. A honeycomb formed body can be obtained byextrusion forming with a ram-type extruder. Though the thus obtainedhoneycomb formed body can be dried in various methods, a combination ofmicrowave drying and hot-air drying or a combination of dielectricdrying and hot-air drying is preferable. There may alternatively beemployed a special method such as freeze drying, reduced-pressuredrying, vacuum drying, and far-infrared radiation drying. Next, thedried honeycomb formed body is cut at both end faces to have apredetermined length.

Next, the plugging step is described. First, a film is disposed on anend face in a masking sub-step. As a film material, a polyester film(No. 631S#25 with film thickness of 50 μm, produced by TeraokaSeisakusho, Co., Ltd.) is used. An adhesive is applied on one surface ofthe film, and the film is applied on an end face of the honeycombstructure. Next, holes are formed in a checkerwise pattern at theopening portions of the cells at an end face where the polyester film isapplied on the honeycomb structure with a laser apparatus capable of NCscanning. When the hole is formed, the periphery of the holes is swollenbecause the film melts.

It is preferable to use, as the main raw material of the pluggingmaterial, 0 to 20% by mass of kaolin having an average particle diameterof 1 to 20 μm, 37 to 40% by mass of talc having an average particlediameter of 5 to 60 μm, 15 to 45% by mass of aluminum hydroxide havingan average particle diameter of 0.5 to 20 μm, 0 to 15% by mass ofaluminum oxide having an average particle diameter of 1 to 20 μm, and 10to 20% by mass of fused silica or quartz having a particle diameter of 1to 200 μm as a cordierite ceramic raw material excellent in thermalresistance and low thermal expansibility from the view point of smalldifference in thermal expansion coefficient with the honeycomb formedbody after firing. Further, the main raw material of the pluggingmaterial preferably has a similar particle size distribution as theabove honeycomb formed body from the viewpoint of small difference infiring shrinkage from the honeycomb formed body. In addition, the mainraw material of the plugging material preferably has a similarcomposition as the raw material of the above honeycomb formed body fromthe viewpoint of bringing the thermal expansion coefficient ratio of ahoneycomb fired body and a plugged material fired body close to 1.

Next, a filling sub-step is described. To a cordierite-forming materialare added water, a binder, a dispersant, and the like, to obtain slurryhaving a viscosity of about 200 dPa·s. The slurry is put in a containerfor plugging. A honeycomb structure with a film having holes in acheckerwise pattern being applied is put in the container under apressure to impregnate the honeycomb structure with the slurry fromholes. After the completion of the impregnation, the honeycomb structureis taken out of the container. Thus, plugged portions for plugging cellsare formed at an end face of the honeycomb structure.

As a suitable method for filling the plugging material up to the sameface as the end face of the honeycomb structure is, for example, amethod in which the plugging material is filled one in the first place,after a tip of the plugging material is moved in the inner direction ofthe cell passages, the plugging material filled is dried to besolidified adequately, and further the plugging material is filled intothe cells. This is based on the finding that, though the tip of theplugging material sometimes moves in the inner direction of the cellpassages by the first filling, the tip of the plugging material hardlymoves even if the additional plugging material is filled into the cells.

It is considered that a phenomenon that the plugging material moves inthe inner direction of the cell passages is due to volume shrinkage ofthe plugging material with water in the plugging material being absorbedby the partition wall portions of the honeycomb structure. Therefore,the tip of the plugging material moves in the inner direction of thecell passages, and the tip is depressed in comparison with the end face.The inventors call this phenomenon surface sink. When the volumeshrinkage of the plugging material is large, not only the tip of theplugging material on the end face side of the honeycomb structure, butalso the tip end of the inner side of the cell passages is sometimesdepressed. Generally, by filling twice, the plugging material can befilled up to the upper surface of the film or higher. However, theplugging material may be filled three times or more.

Though the plugging material can be filled up to almost the same surfaceas the end face of the honeycomb structure, there are a case that theplugging material slightly protrudes and a case that the pluggingmaterial slightly depresses from the end face of the honeycomb structureby the filling of the plugging material. It may be left as it is, orwhen it protrudes, the protruding portion may be removed to give almostthe same surface as the end face. When the tip of the plugging materialdepresses too much, it is preferable to fill the plugging material againto make the depression smaller because soot in exhaust gas is prone todeposit in the depression. In a plurality of the above fillingoperations, it sometimes happens that air is drawn in the pluggingmaterial to form a void inside the plugging material. However, there isno functional problem caused in the plugging material. A void inside theplugging material is also formed by an air bubble which is mixed in theplugging material from the beginning. Therefore, it is also preferableto subject the plugging material to a degassing treatment. Though thesize of the void is varied because it depends on the volume of drawn airand the size of a bubble in the plugging portion, it sometimes reachesabout the size of a cell opening as an easily visible void. Even in sucha case, since the plugged portions and the partition walls are unitarilyjoined, the function of the plugged portions can sufficiently beguaranteed.

In addition, when the tip of the plugging material moves in the innerdirection of cell passages, volume shrinkage of the plugging materialitself is caused. Therefore, there is sometimes caused air holes orvertical and horizontal minute cracks inside the plugged portions.However, since soot does not leak out extremely from the pluggedportions as long as cracks do not pass through with widely opening fromthe tip of the plugged portion to the inner portion, there is nofunctional problem in the plugged portion. Though the size of the cracksis varied, since the plugged portions and partition walls are unitarilyjoined, the function of the plugged portions can sufficiently beguaranteed.

By adequately adding water by means of spraying or the like surroundingthe cell inlet for filling the plugging material of the honeycombstructure before the first filling of the plugging material, thephenomenon that water in the plugging material moves inside thehoneycomb structure is suppressed. Therefore, it is effective insuppressing movement of the tip of the plugging material in a celldirection, and a plurality of filling of the plugging material can beavoided.

In the case that the honeycomb structure is unfired, it is necessary todetermine an adequate volume of water because the partition walls issoftening and deformed if superfluous water is added. There is an effectin suppressing the movement of the tip of the plugging material in adirection of cells by adding a suitable amount of fiber material ofalumina fibers or mullite fibers or a thickener to the pluggingmaterial. Addition of a superfluous amount increases viscosity of theplugging material excessively to make difficult the filling of theplugging material in cell passages. Therefore, it is necessary todetermine an adequate amount of addition.

Further, there is an effect in accelerating volume shrinkage of theplugging material by a method in which pressure is applied inside thecells by air or the like from the opposite side of the cells during theabove filling of the plugging material. Therefore, surface sink can besuppressed by filling the plugging material with being pressurized.

Next, in order to dry the plugging material, the plugged end face of thehoneycomb structure is subjected to hot air having a temperature of 160°C. for about five minutes without peeling the film. Drying can beperformed with a hot plate. The same is conducted also for the other endface to form the plugging portions at both the end faces. Then, thehoneycomb structure is fired to obtain a cordierite based pluggedhoneycomb structure.

Partition walls of the cordierite based plugged honeycomb structureobtained practically based on the above method had a porosity of 67%,which was measured with a mercury porosimeter, and an average porediameter of 27 μm. Each cell had a rectangular shape, partition wallshad a thickness of about 0.3 mm, and the cell pitch was about 1.6 mm.The filter had a diameter of about 191 m and a length of about 200 mm.The plugged portions had a length of about 0.3 mm from an end face ofthe filter toward the inner direction of cell passages. Incidentally, adesired additive may be added to the ceramic material as necessary.Examples of the additive include the binders, dispersants, and poreforming materials which were specifically described above.

The plugged honeycomb structure thus produced on the basis of thepresent invention, as a DPF, was held under pressure by a ceramic mat(Commercial name: Interam Mat, produced by 3M) in a metallic case. Then,both end faces of the DPF was fixed with a fixing member, the fixingmember was welded to the metallic case, and a cone was connected to themetallic case to produce a converter assembly. A converter assemblyemploying a conventional plugged honeycomb structured DPF was alsoproduced. Then, each of the converter assemblies produced was connectedto an exhaust gas system of a practical diesel engine (with adisplacement of about 5 liters), and exhaust gas was sent to conduct aheating-cooling test. Each of the converter assemblies was decomposedand investigated. A trace of leakage of black smoke was recognized in aninterface portion between a plugged portion of an end face of the DPFand a partition wall in the converter assembly of the conventionalstructure, while it was not recognized in the converter assembly of thepresent invention.

Further, as a result of washing with high pressure water using a RockyWasher from an exhaust gas outlet side of the DPF, a part of the pluggedportions in the exhaust gas outlet end face of the DPF was depressedfrom the end face toward the inner portion of the cell passages and apart of the plugged portions in the exhaust gas inlet end face of theDPF was protruded from the end face in the converter assembly of theconventional structure. On the other hand, as a result of the washingthe DPF of the present invention with high pressure water in the samemanner, there was no problem observed in the plugged portion at both endfaces of the DPF. Further, in a washing operation with high pressureair, a similar effect was confirmed.

FIG. 7 shows a photograph of a parallel section of a plugged honeycombstructured DPF (Example) produced by the method of the above presentinvention, and FIG. 8 shows a photograph of a parallel section of aplugged honeycomb structured DPF (Comparative Example) produced by theconventional method. From these photographs, it is understood that theplugged portion and the partition wall surrounding the plugged portionare unitarily formed in the plugged honeycomb structures DPF of Examplesince there is no interface line observed between the plugged portionand the partition wall and a phase extending over both the pluggedportion and the partition wall is observed, while it is understand thatthe plugged portion and the partition wall surrounding the pluggedportion are not unitarily formed in the plugged honeycomb structures DPFof Comparative Example since there is an interface line observed betweenthe plugged portion and the partition wall. Incidentally, the pluggedhoneycomb structured DPF obtained in Comparative Example was produced inthe same manner as in Example except for the plugging step. That is,Comparative Example refers to a method described in JP-A-57-42316, and ahoneycomb structure having a diameter of 144 mm, a length of 150 mm, anda partition wall thickness of about 0.4, and a cell pitch of about 2.5was formed and dried in completely the same manner as in Example andthen fired without being plugged. Next, a honeycomb structure (firedbody) was subjected to plugging using a plugging material slurryproduced in completely the same manner as in Example and fired again tofire the plugging material. In this case, since the honeycomb structurewas already a fired body, large shrinkage was not caused in partitionwalls during the second firing, and large difference is generated inshrinkage between the partition walls and the plugged portion. Even byobserving the gap between the plugged portion and the partition wallfrom above the end face of the honeycomb structure, an interface seen inFIG. 8 cannot be observed, and the interface was not found until thesection of the plugged portion was observed as in FIG. 8. As anadditional confirmation test, a method referring to JP-A-2002-173381 wastested, and interface similar to that of the photograph of FIG. 8 wasobserved between the plugged portion and the partition wall. In thiscase, it is considered that the difference in shrinkage and thedifficulty in adaptation due to low compatibility between the pluggedportion and the partition wall influenced. Thus, in a schematic viewshown in prior art, it is drawn as if an interface is not present in thecontact portion between a partition wall and a plugged portion. However,it is understood that an interface is actually present in the contactportion between a partition wall and a plugged portion in conventionaltechnique as shown by the result of observing section of FIG. 8.

INDUSTRIAL APPLICABILITY

As described above, since a plugged portion and a partition wallsurrounding the plugged portion are unitarily formed in a pluggedhoneycomb structure of the present invention, the honeycomb structurehas high strength and thermal shock resistance between the partitionwall and the plugged portion and useful for various usage as a filter orthe like of a DPF, etc. In addition, by a production method of thepresent invention, a plugged honeycomb structure having unitarily formedplugged portions and partition walls surrounding the plugged portionscan easily be produced.

1. A plugged honeycomb structure comprising: partition walls disposed soas to form a plurality of cells extending from one end face to the otherend face in an axial direction, and plugged portions disposed so as toplug the cells at one of the end faces, wherein said plugged portionsand said partition walls surrounding said plugged portions are unitarilyformed, and have no interface line.
 2. A plugged honeycomb structureaccording to claim 1, wherein a length of said plugged portions in alongitudinal direction of the cells is ten times as long as a cell pitchor less.
 3. A plugged honeycomb structure according to claim 1, whereinthe length of said plugged portions in the longitudinal direction of thecells is two times as long as a thickness of said partition walls ormore.
 4. A plugged honeycomb structure according to claim 2, wherein thelength of said plugged portions in the longitudinal direction of thecells is two times as long as a thickness of said partition walls ormore.
 5. A method for producing a plugged honeycomb structure,comprising: a forming step of forming a forming raw material containinga solid particle (A) into a honeycomb formed body provided with apartition wall-formed body disposed so as to form a plurality of cellsextending from one end face to the other end face in an axial direction,a plugging step of filling a plugging material containing a solidparticle (B) into end portions of cells of the honeycomb formed body toform a plugged honeycomb formed body, and a firing step of firing theplugged honeycomb formed body to form a plugged honeycomb structureprovided with partition walls formed out of the partition wall-formedbody and plugged portions formed out of the plugging material; whereinthe solid particle (A) and the solid particle (B) can unitarily beformed mutually in the firing step with no interface line, and adifference between rate of dimensional change (%) upon forming theplugged portions out of the plugging material and rate of dimensionalchange (%) upon forming the partition walls out of the partitionwall-forming material is controlled to be 7% or less within thetemperature range of the firing in the firing step.
 6. A method forproducing a plugged honeycomb structure according to claim 5, whereinthe method further comprises a cooling step of cooling the pluggedhoneycomb structure, and a ratio of thermal expansion coefficient of theplugged portions to thermal expansion coefficient of the partition wallsis made to be 0.3 to 3.0 in the temperature range of the cooling in thecooling step.
 7. A method for producing a plugged honeycomb structureaccording to claim 5, wherein said plugging step includes a step offilling the plugging material from one end face of the honeycombstructure with pressurizing inside the cells from the other end face. 8.A method for producing a plugged honeycomb structure according to claim5, wherein the forming raw material contains a binder, and the pluggingmaterial contains a binder compatible with the binder contained in theforming raw material.
 9. A method for producing a plugged honeycombstructure according to claim 5, wherein the forming raw materialcontains a binder, and after the forming step, an end face of thehoneycomb formed body is heated at 200° C. or more to remove at least apart of the binder, followed by conducting the plugging step at the endface.
 10. A method for producing a plugged honeycomb structure accordingto claim 5, wherein the forming raw material is a slurry containing adispersion medium (C), and the plugging material is a slurry containinga dispersion medium (D) compatible with the dispersion medium (C).
 11. Amethod for producing a plugged honeycomb structure according to claim10, wherein the honeycomb formed body is dried, subsequently, thepartition wall-formed body is impregnated with the dispersion medium (C)in the vicinity of an end face, followed by conducting the plugging stepat the end face.
 12. A method for producing a plugged honeycombstructure according to claim 10, wherein the dispersion medium (C) andthe dispersion medium (D) are hydrophobic, and the plugging step isconducted under the condition that temperature of the partitionwall-formed body is higher than the normal temperature in the vicinityof an end portion.